Low density polyethylene (LDPE) produced by a high-pressure radical process is an ethylene polymer having high melt strength (MS). However, fluidized activation energy (Ea) is large, and temperature dependency of melt viscosity is large. Therefore, the polymer has high MS in only a specific molding processing temperature range, and it was necessary to select a processor suitable for a polymer to be processed in molding processing.
Furthermore, an ethylene polymer obtained using a Ziegler catalyst or a metallocene catalyst has small Ea, but MS is low. Therefore, there was the problem in molding processability.
For example, (a) an ethylene-α-olefin copolymer having long chain branch obtained using a specific metallocene catalyst under specific polymerization conditions (for example, see Patent Document 1), and (b) an ethylene-α-olefin copolymer having Ea of 60 kJ/mol or more obtained using a specific metallocene catalyst (for example, see Patent Document 2) are proposed as an ethylene polymer having improved molding processability. However, those ethylene copolymers have large Ea and large temperature dependency of melt viscosity, similar to LDPE. Therefore, it was necessary to strictly control temperature in molding processing.
An ethylene polymer having low Ea and high MS is preferred to have stable processability in a wide molding processing temperature range. Such a polymer includes an ethylene polymer produced using a Cr-based catalyst. However, in the present inventors' investigations, there was the problem on thermal stability such as yellowing at the time of melt processing due to that terminal vinyl group is present in the ethylene polymer in an amount of 0.3 or more per 1,000 carbon atoms. Thus, an ethylene polymer having small number of unsaturated bonds such as terminal vinyl group and excellent thermal stability, and additionally having low Ea and excellent molding processability in a wide molding processing temperature range has been desired.
Furthermore, the ethylene polymer produced using a Cr-based catalyst has the problem that a molecular weight distribution is wide, and as a result, mechanical strength of a molding is low. A catalyst for producing an ethylene polymer having excellent mechanical strength and excellent molding processability under wide molding processing conditions, and a production method using the same have been desired.
Patent Document 1: U.S. Pat. No. 5,272,236
Patent Document 2: JP-A-2004-292772